Friction kernels for the relative dynamics of ion pairs in water

Abstract

The dynamic properties of Cl−–Cl−, Cl−–Na+, and Na+–Na+ ion pairs in water are investigated. It is assumed that the interionic separations obey the generalized Langevin equation. The random forces on the ion pairs are calculated from constrained molecular-dynamics simulations at three interionic distances for each ion pair. The Gaussianity of the random forces is checked. The time-dependent friction kernels for the relative motions are determined from the random forces by using the fluctuation–dissipation theorem. The dependence of the friction kernels on both the solute species and the intersolute separation is explored. Special attention is paid to the relationship of the friction kernels with the characteristics of the configurations of the solvent molecules in the neighborhood of the ions. The coupling between the intramolecular motions of water and the relative ionic motions is discussed. The association–dissociation processes for the three ion pairs are analyzed within the framework of the Kramers and Grote–Hynes theories for activated barrier crossing kinetics.